1. Field of the Invention
The present invention relates to a portable, compact radiation clinical thermometer for measuring a temperature upon insertion in an external ear canal.
2. Description of the Prior Art
Recently, a pen type electronic clinical thermometer has been widely used in place of a glass clinical thermometer.
This electronic clinical thermometer is not fragile, can perform a digital display which is easy to read, and can generate an alarm sound such as a buzzer sound for signaling the end of temperature measurement. However, this clinical thermometer requires about 5 to 10 minutes for temperature measurement, i.e., substantially the same length of time as that required by a glass clinical thermometer. This makes a user feel that body temperature measurement is cumbersome. Such a long measurement time is based on a method of inserting a sensor portion in an armpit or a mouth and bringing it into contact with a portion to be measured. A measurement time is prolonged due to the following two reasons:
(1) A skin temperature at an armpit or a mucous membrane temperature in a mouth is not equal to a body temperature prior to temperature measurement, and gradually reaches the body temperature after the armpit or the mouth is closed.
(2) Since the sensor portion of the clinical thermometer has been cooled down to an ambient temperature, when it is inserted in a portion to be measured, the temperature of the portion is further lowered.
Temperature measurement of a conventional clinical thermometer will be described with refence to FIG. 1.
FIG. 1 shows temperature measurement curves of a contact type electronic clinical temperature. In FIG. 1, temperature measurement time is plotted along the axis of abscissa and measurement temperatures are plotted along the axis of ordinate. A curve H represents a temperature curve of an armpit as a portion to be measured; and a curve M, a measurement temperature curve obtained by the clinical thermometer. Accordingly, the skin temperature of the armpit is 36.degree. C. or less at measurement start time t.sub.1, and the temperature of a clinical thermometer sensor portion is cooled to 30.degree. C. or less. When the sensor portion is inserted in the armpit in this state, and the armpit is closed, the measurement temperature represented by the curve M of the sensor portion is quickly raised. However, the temperature represented by the curve H of the armpit begins to rise gradually toward an actual body temperature T.sub.b after it is cooled by the sensor portion to a temperature at time t.sub.2. The two temperature curves H and M coincidentally rise from time t.sub.3 when the sensor portion is warmed to the skin temperature of the armpit. As described above, however, it takes about 5 to 10 minutes for the curve to reach the actual body temperature. As is known, a method of measuring a body temperature is performed in practice as follows. Measurement is performed from time t.sub.1 at predetermined intervals. The measurement values are compared with each other, and maximum values are sequentially stored. At the same time, a difference between the measurement values is sequentially checked. The instant when the difference becomes smaller than a predetermined value is set at time t.sub.4, and the temperature measurement is stopped. Thus, the greatest value at this time is displayed as a body temperature (e.g., Japanese Patent Laid-Open (Kokai) No. 50-31888).
In consideration of the above-described reasons (1) and (2), conditions for performing body temperature measurement within a short period of time are: selection of a portion having a body temperature prior to measurement, and an actual measurement without bringing a cooled sensor portion into contact with the portion to be measured.
A drum membrane is, therefore, selected as a portion having a body temperature prior to measurement, and a radiation clinical thermometer is proposed as a clinical thermometer for measuring the temperature of the portion in a nontact manner (e.g., U.S. Pat. No. 3,282,106).
The principle of a radiation thermometer on which the above radiation clinical thermometer is based will be described below.
A radiation thermometer is based on a law of physics, i.e., "all objects emit infrared radiation from their surfaces, and the infrared radiation amounts and the spectral characteristics of the objects are determined by their absolute temperatures as well as their properties and states of their finished surfaces." This law will be described with reference to the following laws.
The Planck's law states a relationship between the radiant intensity, spectral distribution, and temperature of a blackbody as follows: EQU W(.lambda., T)=2.pi.c.sup.2 h/.lambda..sup.5 (e.sup.hc/k.lambda.T -1).sup.-1( 1)
where
W(.lambda.,T): spectral radiant emittance [W/cm.sup.2 .multidot..mu.m] PA1 T: absolute temperature of blackbody [K] PA1 .lambda.: wavelength of radiation [.mu.m] PA1 c: velocity of light=2.998.times.10.sup.10 [cm/sec] PA1 h: Planck's constant=6.625.times.10.sup.-34 [W.sec.sup.2 ] PA1 k: Boltzmann constant=1.380.times.10.sup.23 [W.sec/K] PA1 .gamma.: reflectance of object PA1 V.sub.d : output voltage from infrared amplifier 4a PA1 S: light-receiving area of infrared sensor 3a PA1 R: sensitivity of infrared sensor PA1 A: gain of infrared amplifier 4a
FIG. 3 shows the Planck's law. As is apparent from FIG. 3, as the temperature of the blackbody rises, the radiation energy is increased. In addition, the radiation energy varies depending on wavelengths. The peak value of the radiant emittance distribution shifts to the short wavelength side with an increase in temperature, and radiation occurs over a wide wavelength band.
Total energy radiated from the blackbody can be obtained by integrating W(.lambda.,T) given by equation (1) with respect to .lambda. from .lambda.=0 to .lambda.=.infin.. This is the Stefan-Boltzmann law. ##EQU1## W.sub.1 : total energy radiated from blackbody [W/cm.sup.2 ] .sigma.: Stefan-Boltzmann constant=5.673.times.10.sup.12 [W/cm.sup.2.deg.sup.4 ]
As is apparent from equation (2), the total radiation energy W.sub.1 is proportional to a power of four of the absolute temperature of the blackbody light source. Note that equation (2) is obtained by integrating the infrared radiation emitted from the blackbody with respect to all the wavelengths.
All the above-described laws are derived from the blackbody having an emissivity of 1.00. In practice, however, most objects are not ideal radiators, and hence have emissivities smaller than 1.00. For this reason, the value obtained by equation (2) must be corrected by multiplying a proper emissivity. Radiation energy of most objects other than the blackbody can be represented by equation (3): ##EQU2## .epsilon.: emissivity of object
Equation (3) represents infrared energy which is radiated from an object and incident on an infrared sensor. However, the infrared sensor itself emits infrared radiation in accordance with the same law described above. Therefore, if the temperature of the infrared sensor itself is T.sub.0, its infrared radiation energy can be given as .sigma.T.sub.0.sup.4, and energy W obtained by subtracting radiation energy from incident energy is given by equation (4): EQU W=.sigma.(.epsilon.T.sup.4 +.gamma.T.sub.a.sup.4 -T.sub.0.sup.4)(4)
T.sub.a : ambient temperature of object
Since the transmittance of the object to be measured can be regarded as zero, .gamma.=1-.epsilon. can be established.
In equation (4), the infrared sensor is considered to be ideal and hence has an emissivity of 1.00.
In addition, assuming that the infrared sensor is left in an atmosphere of an ambient temperature T.sub.a so that the infrared sensor temperature T.sub.0 is equal to the ambient temperature T.sub.a, equations (4) can be rewritten as equation (5): ##EQU3##
FIG. 2 shows a basic arrangement of a conventional radiation thermometer. The arrangement will be described below with reference to FIG. 2.
A radiation thermometer comprises an optical system 2, a detecting section 3, an amplifying section 4, an operating section 5, and a display unit 6.
The optical system 2 is constituted by a focusing means 2a for efficiently focusing infrared radiation from an object L to be measured, and a filter 2b having transmission wavelength characteristics. A cylindrical member having an inner surface plated with gold is used as the focusing means 2a. A silicon filter is used as the filter 2b.
The detecting section 3 is constituted by an infrared sensor 3a and a temperature-sensitive sensor 3b. The infrared sensor 3a converts infrared radiation energy obtained by subtacting its own radiation energy from incident infrared radiation energy focused by the optical system 2 into an electrical signal, i.e., an infrared voltage v.sub.s. In addition, the temperature-sensitive sensor 3b is arranged near the infrared sensor 3a to measure the temperature of the infrared sensor 3a and its ambient temperature T.sub.0, and outputs a temperature-sensitive voltage v.sub.t. A thermopile and a diode are respectively used as the infrared sensor 3a and the temperature-sensitive sensor 3b.
The amplifying section 4 comprises an infrared amplifier 4a, constituted by an amplifying circuit and an A/D converter for converting an output voltage from the amplifying circuit into digital infrared data V.sub.d, for amplifying the infrared voltage v.sub.s output from the thermopile, and a temperature-sensitive amplifier 4b, constituted by an amplifying circuit and an A/D converter for converting an output voltage from the amplifying circuit into digital temperature-sensitive data, for amplifying the temperature-sensitive voltage v.sub.t as a forward-biased voltage from the temperature-sensitive sensor 3b, i.e., the diode.
Two signals V.sub.d and T.sub.0 from the amplifying section 4 are then converted into temperature data T, and are displayed on the display unit 6. The operating section 5 comprises an emissivity input means 5a for setting an emissivity .epsilon. of the object L, and an operating circuit 5c for performing an operation based on equation (5).
With the above-described arrangement, temperature measurement of the object L can be performed by a noncontact scheme. An operation of this temperature measurement will be described below.
The object L emits infrared radiation, and its wavelength spectrum distribution covers a wide wavelength range, as shown in FIG. 3. The infrared radiation is focused by the focusing means 2a, transmitted through the filter 2b having the transmission wavelength characteristics, and reaches the infrared sensor 3a.
Other infrared radiation energies reach the infrared sensor 3a. One is infrared radiation energy emitted from a certain object near the object L, which is reflected by the object L and is then transmitted through the filter 2b and reaches the infrared radiation energy. Another is infrared radiation energy emitted from the infrared sensor 3a or a certain object near the sensor 3a, which is reflected by the filter 2b and reaches the sensor 3a. Still another is infrared radiation energy which is emitted from the filter 2b and reaches the sensor 3a.
The infrared radiation energy from the infrared sensor 3a can be represented by equation (3). In this case, .epsilon.=1.00. That is, to measure the temperature of the infrared sensor 3a itself is to indirectly measure the infrared radiation energy from the infrared sensor 3a. For this purpose, the temperature-sensitive sensor 3b is arranged near the infrared sensor 3a and measures the temperature of the infrared sensor 3a and the ambient temperature T.sub.0. The infrared sensor 3a converts the infrared radiation energy W obtained by subtracting infrared radiation energy emitted therefrom from infrared radiation energy incident thereon into an electrical signal. Since the infrared sensor 3a employs a thermopile, it outputs the infrared voltage v.sub.s proportional to the infrared radiation energy W.
In this case, the infrared voltage v.sub.s as an output voltage from the infrared sensor 3a corresponds to a value obtained by multiplying the product of the infrared radiation energy W per unit area and a light-receiving area S of the infrared sensor 3a by a sensitivity R. The infrared data V.sub.d as an output voltage from the infrared amplifier 4a corresponds to a value obtained by multiplying the infrared voltage v.sub.s from the infrared sensor 3a by a gain A of the infrared amplifier 4a. EQU V.sub.s =R.multidot.W.multidot.S EQU V.sub.d =A.multidot.v.sub.s
Since the above equations can be established, equation (5) can be expressed as equation (6) as follows: EQU V.sub.d =.epsilon..multidot..sigma.SRA(T.sup.4 -T.sub.0.sup.4)(6)
where
Generally, equation (6) is simplified by setting K.sub.1 =.sigma.SRA, and hence the temperature T of the object L is calculated according to equation (7). ##EQU4##
A thermal infrared sensor used for a conventional radiation thermometer has no wavelength dependency. However, a transmission member such as a silicon or quarts filter is arranged as a window member on the front surface of a can/package in which the infrared sensor is mounted due to the following reason. Since infrared radiation from an object has the wavelength spectrum distribution shown in FIG. 3, such a filter is used to transmit only infrared radiation having a main wavelength band therethrough so as to reduce the influences of external light. Each of the above-described transmission members has unique transmission wavelength characteristics. A proper transmission member is selected on the basis of the temperature of an object to be measured, workability and cost of a transmission member, and the like.
FIG. 4 shows the transmittance of a silicon filter as one of the transmission members. The silicon filter shown in FIG. 4 transmits only infrared radiation having a wavelength band from about 1 to 18 [.mu.m] therethrough, and has a transmittance of about 54%.
As described above, an infrared sensor with a filter has wavelength dependency, i.e., transmits infrared radiation having a specific wavelength band because of the filter as a window member although the sensor itself is a temperature sensor and has no wavelength dependency.
Therefore, equation (5) obtained by integrating infrared radiation energy incident on the infrared sensor with a filter with respect to all the wavelengths cannot be applied to the infrared sensor with a filter for transmitting infrared radiation having a specific wavelength band, and an error is included accordingly.
Furthermore, in the conventional arrangement, the sensitivity R of the infrared sensor is used as a constant. In practice, however, the sensitivity R of the infrared sensor varies depending on the infrared sensor temperature T.sub.0. FIG. 5 shows this state. In FIG. 5, the sensitivity R is obtained by actually measuring the output voltage v.sub.s from a thermopile as an infrared sensor by using a blackbody, and the infrared sensor temperature T.sub.0 is changed to plot changes in sensitivity R at the respective temperatures. As a result, it is found that the temperature dependency of the sensitivity R can be approximated to a straight line as represented by equation (8): EQU R=.alpha.{1+.beta.(T.sub.0 -T.sub.m)} (8)
where
.alpha.is the sensitivity R as a reference when T.sub.0 =T.sub.m, T.sub.m is a representative infrared sensor temperature, e.g., an infrared sensor temperature measured in a factory, and .beta. represents a coefficient of variation. In this case, a coefficient of variability per 1 [deg] is -0.3 [%/deg]. The variation in sensitivity R described above inevitably becomes an error.
The coefficient of variation .beta. is influenced by the manufacturing conditions of a thermopile, and can be decreased by increasing the purity and process precision of the thermopile. However, thermopiles on the market which are mass-produced have the above value.
A radiation thermometer, however, is normally designed to measure high temperatures, and has a measurement range from about 0.degree. to 300.degree. C. and measurement precision of about .+-.2.degree. to 3.degree. C. Therefore, errors due to the above-described filter characteristics, variations in sensitivity of an infrared sensor, and the like are neglected, and hence no countermeasure has been taken so far. When measurement conditions as a clinical thermometer are taken into consideration, however, a temperature measurement range may be set to be as small as about 33.degree. C. to 43.degree. C., but .+-.0.1.degree. C. is required for temperature measurement precision. Therefore, if the above-described radiation thermometer is used as a clinical thermometer, temperature measurement precision must be increased by taking countermeasures against errors due to the filter characteristics and the variations in sensitivity of infrared radiation.
A radiation clinical thermometer disclosed in U.S. Pat. No. 4,602,642 employs the following system as a countermeasure.
This radiation clinical thermometer comprises three units, i.e., a probe unit having an infrared sensor, a chopper unit having a target, and a charging unit. In addition, a heating control means for preheating the infrared sensor and the target to a reference temperature (36.5.degree. C.) of the external ear canal is provided, and is driven by charged energy from the charging unit. When a body temperature is to measured, the probe unit is set in the chopper unit, and the probe unit having the infrared sensor and the target are preheated by the heating control means. In this state, calibration is performed. Thereafter, the probe unit is detached from the chopper unit and is inserted in an external ear canal to detect infrared radiation from a drum membrane. A body temperature measurement is performed by comparing the detected infrared radiation with that from the target.
Temperature measurement precision is increased by the above-described system for the reasons to be described below.
According to this system, various error factors are eliminated by preheating the probe unit having the infrared sensor and the target to a reference temperature (36.5.degree. C.) close to a normal body temperature by using the heating control means. That is, by heating the probe to the reference temperature higher than a room temperature and keeping the infrared sensor at a constant temperature regardless of ambient temperatures, sensitivity variations of the infrared sensor can be eliminated, and hence its error can be neglected. In addition, calibration is performed so as to set the reference temperature of the target to be close to a body temperature to be measured, and a comparative measurement is then performed so that errors and the like due to the filter characteristics are reduced to a negligible level. Furthermore, since the probe is preheated to a temperature close to a body temperature, the problem of the conventional measurement system can be solved, i.e., the problem that when a cool probe is inserted in an external ear canal, the temperatures of the external ear canal and the drum membrane are lowered because of the probe, so that correct body temperature measurement cannot be performed.
The above-described radiation clinical thermometer disclosed in U.S. Pat. No. 4,602,642 is excellent in temperature measurement precision. However, since this thermometer requires a heating control unit with high control precision, its structure and circuit arrangement become complicated, thereby increasing the cost. In addition, it requires a long stable period to preheat the probe and the target and control their temperatures to a predetermined temperature. Moreover, since the heating control unit is driven by a relatively large-power energy, a large charging unit having a power source cord is required. Therefore, the above-described system cannot be applied to a portable clinical thermometer using a small battery as an energy source.